In the classical railroad air brake system, as developed from the Westinghouse air brake, the brake air line which passes from the locomotive and then from car to car down the length of the train, provides two basic functions.
First, it is used to charge compressed air tanks in the railroad cars. The air stored in these tanks provides the energy needed to apply the brake shoes when a brake application is required. When the train is running normally, and no brake application is needed, a high pressure, typically 90 pounds for freight trains and 110 pounds for passenger trains, exists in the brake air line. The tanks in the cars are charged to the same pressure as the air in the brake air line.
Second, when a brake application is required, some of the air is vented from the valve in the locomotive which supplies the brake air line, causing the pressure in the brake air line to be reduced. In the cars of the train, this reduction of pressure is used as a signal to apply the brakes. In this event, valving in the cars utilizes the compressed air in the tanks to supply air to brake cylinders which apply force to the brake shoes so that the brakes are applied.
Although this classical air brake system was an enormous improvement over the art prior to it, it nevertheless had some features where improvement was possible. For one thing, the time needed for a pressure decrement to propagate down the line of cars in a long freight train is quite long, about a minute for a mile-long train. Hence, when a brake application is required, it takes some time before all the brakes in the train are applied. This is the case for both normal and emergency brake applications.
There are also some operational difficulties due to the fact that the same compressed air line is used both for charging the air tanks in the cars, and for signalling brake application. When a brake application is made, some of the air in the air tanks is depleted. At the same time, the pressure in the brake air line is reduced to signal the brake application. The air in the air tanks cannot be recharged to its initial pressure while the brake line air pressure is low for applying brakes.
One approach to alleviating these problems is to provide a radio link so that when a signal originates in a lead locomotive to apply brakes, a radio signal is transmitted which is received at some distance down the line of cars. Where the signal is received, it causes local venting of the brake line, so that brakes are applied more rapidly. The WABCO EPIC brake system may be operated with a radio link for this purpose. (Registered trademark of Westinghouse Airbrake Company)
Another approach which is embodied in several systems which are incompatible with each other, but in which each is compatible with the classical air brake system, is to provide electrical trainlines, which are electric cables connected from car to car down the length of the train. These are provided in addition to the classical air line. These cables carry signals which control the operation of various air valves in the cars. These provide improved speed of response, and provide various operational improvements.
Several systems employ three electrical trainlines, but use them in different ways. These lines are identified as the application line, the release line, and the emergency line. Three systems, each using three such lines, are discussed here. These are the WABCO MC30A/CS-1, the WABCO 26-C/CS-2, and the New York Air Brake system, the NYAB PS-68.
In all three systems, the signals on the lines are, at any time, on or off, these being referred to as digital signals. In all three systems, a signal on the application line causes an immediate normal brake application in all the cars, or an immediate increase of application of brakes in all the cars. In all three systems, a signal on the emergency line causes air to be dumped rapidly from the brakeline in all the cars to provide an immediate emergency brake application.
The release line is used differently in the different systems. In the CS1 system, a signal on the release line causes high pressure air to be admitted to the brake line causing a graduated release of the brakes. In the CS2 system, a signal on the release line prevents release of air from the brake cylinders in the cars so the brake line pressure can be increased without releasing the brakes.
In the CS1 system, high pressure air for increasing the brake line pressure in the cars of the train is obtained from a second line which carries compressed air down the length of the train.
In the CS1 system, braking forces can be modulated on and off (within certain limits) by modulating the brakeline pressure. The air tanks in the cars are kept charged by the second compressed air line, and the amount of brake application is proportional to the decrement in brakeline pressure compared to the pressure in a reference reservoir.
The CS1 system has, in the locomotive, an added system which has a diaphragm which makes a comparison between a pressure defined by the position of the brake control lever, and the pressure in the brake line in the locomotive. Motion of the diaphragm in either direction is detected by one of two microswitches, one of which controls a signal applied to the application trainline, and one of which controls a signal applied to the release trainline.
These signals are used, in the cars of the train, to adjust the brakeline pressure so it follows the pressure established by the position of the brake control lever. A signal on the application trainline causes a release of air pressure at a known rate from the brake line in all the cars. This causes application of brakes, or causes increased application of brakes. A signal on the release line causes air to flow at a known rate from the second compressed air line in each of the cars to the brake line to release or cause a reduction of the forces on the brake shoes.
In all of these systems, compatibility is maintained with the classical airbrake system to the extent that braking function is provided by the classical air brake line and the braking equipment in the cars. Hence, a car equipped with one of these systems can be included in a train not so equipped, so it can be taken to a customer location, to a repair shop, etc.
The various systems which use electrical trainlines to provide enhanced braking function, however, are incompatible with each other. This is one disadvantage of the systems described above. A locomotive equipped for one system cannot provide the appropriate electrical signals to a train of cars equipped with a different system. Another disadvantage of the above systems is the use of microswitches which in some systems are used to determine the position of the brake lever, or, in the CS1 system, are used to provide trainline signals based on diaphragm position.
An additional disadvantage of the these systems is that in some cases, a brake application may be made which is so small that when a release signal is given, the brakes do not release.
The present invention addresses these and other disadvantages of the systems described above, while utilizing much of the rolling stock equipped with these systems.